Add plugin validation to release pre-flight checks

[b-coman]

Context

Anthropic provides a built-in plugin validator (`claude plugin validate .`) that checks plugin.json, skill/agent/command frontmatter, hooks.json syntax, and schema conformance. We are not running it as part of our release process.

Additionally, the Agent Skills ecosystem provides a cross-tool validator (`skills-ref validate`) and a community linter (`claudelint`) that catch broader issues.

Problem

Without automated validation:

• Frontmatter typos or schema violations ship silently
• Skills may fail to load in Claude Code without any pre-release signal
• Format drift from Anthropic's evolving spec goes unnoticed
• Our release script (`scripts/release.sh`) runs Layer 1 pre-flight checks but doesn't include plugin format validation

Deliverables

• Add `claude plugin validate .` to `scripts/release.sh` pre-flight checks (runs before version bump)
• Evaluate and optionally add `skills-ref validate` (pip package from agentskills.io) as a second check
• Document the validation step in CLAUDE.md release section

Acceptance criteria

• `bash scripts/release.sh --dry-run patch "test"` runs plugin validation and reports results
• Validation failure blocks the release (non-zero exit code propagated)
• Current repo passes validation cleanly (fix any issues found, coordinated with Align skill and agent frontmatter with Anthropic's current spec #7)

Dependencies

• Blocked by: Align skill and agent frontmatter with Anthropic's current spec #7 (frontmatter alignment — fix format issues before adding the validator that catches them)

References

• `scripts/release.sh` — current release script
• Claude Code Plugins Reference
• `claude plugin validate` — built-in CLI command
• skills-ref validation library
• claudelint (community)